This invention relates to glass fibers having a coating and/or cladding. More particularly, the present invention relates to glass fibers having a thin layer of an organosilsesquioxane polymer which serves as a coating and/or cladding.
During the past decade, considerable interest has been generated in the use of glass fibers for fiber optic applications. In these applications, it is essential that the fiber selected guide light and evidence mechanical integrity, even under adverse temperature and humidity conditions. Thus, as an example, high cost single mode silica-based fibers commonly employed in under sea cables have a silica-based cladding and require a water barrier to protect the fiber from the adverse effects of humidity. Heretofore, workers in the art have attempted to attain this end by the use of an acrylate coating which is applied to the fiber. The prime disadvantage encountered with this material resides in the fact that the coating loses its protective attributes under high temperature and high humidity conditions, for example, at 90.degree. C. and 90% humidity. Efforts to obviate this limitation focused upon the use of silicon nitride or silicon carbide coatings. These materials did not suffer from the temperature humidity limitations of the acrylates; however, they are known to be very brittle. Accordingly, cracking can be commonly encountered, so exposing the underlying fiber with a cracked coating to moisture and/or elevated temperatures which leads to degradation of fiber characteristics. Thus, the search has continued for suitable coatings for silica-based fibers which maintain strength characteristics at elevated temperatures and humidity. There is a similar need for the mechanical protection of silica-based fibers to be used in applications in which strength is the primary design criterion, including their use in high-strength composites.
To be useful as a fiber cladding, a material must have an appropriately lower index of refraction with respect to the fiber core so as to guide light within the core. In those cases in which the cladding is applied to the fiber core an additional requirement is that the cladding material have good adhesive characteristics. One current difficulty is that much of present day fiber optics technology, including the availability of low cost light sources and detectors, depends on a silica-based material for the light-guiding fiber core and the number of ways to produce a cladding with an index less than that of SiO.sub.2 is quite limited. They include fluorine doped silica, an expensive process used in demanding applications (e.g. undersea fiber optic cables) and the organic or linear siloxane polymers used in the less stringent plastic-coated-silica applications. There is, however, a need for low cost applied claddings with good adhesive qualities and an appropriate lower index of refraction as compared to silica for fiber optic applications in demanding environments which exceed the capabilities of the organic or linear siloxanes.